A drive circuit is provided for energizing liquid crystal displays and more particularly for providing a multiple digit liquid crystal display having a high operation margin.
Liquid crystal displays generally include a plurality of digits each including a plurality of segments. The segments are selectively energized in each digit to display a composite number or symbol. Prior liquid crystal displays have utilized driving means for applying constant voltage levels between electrodes for each selected segment and each number or symbol and controlling the voltage on or off to actuate selected segments. This scheme requires driver circuitry which is relatively expensive because of the large number of circuit paths including driver components required to individually control each segment in each digit. Alternatively AC power has been utilized to obtain display voltage levels between digit electrodes and selected segment electrodes, wherein like segment electrodes in different digits are energized simultaneously to reduce required segment driver circuit components. A problem arises in the latter type of scheme inasmuch as nonselected common segments in adjacent digits may appear in "ghost" display if sufficient segment electrode voltage is applied to obtain optimum contrast at selected segments in the display. Moreover, once optimum contrast is obtained it has been found difficult to maintain source regulation sufficiently fine to retain optimum contrast over the entire range of display excitation conditions. There is, therefore, a need for a liquid crystal display combining high operation margin, high contrast, low ghost characteristics and lack of complexity in the segment driver portion of the driver circuit.